1. Field of the Invention
The present invention relates to a scroll member used in a positive displacement device, such as a compressor, pump or expander, and a method for manufacturing the scroll member.
2. Description of the Related Art
In a positive displacement device, e.g., a scroll compressor, processes of suction, compression, and discharge can be executed simultaneously in a plurality of chambers. Accordingly, driving torque fluctuates only slightly, the level of noise or vibration during operation if low, and the compression efficiency is improved. Thus, the scroll compressor may be utilized for gas compression in the cooling system of refrigerators or freezers, and in the air-conditioning system of air conditioners, etc.
In the scroll compressor, a scroll compressing unit is disposed in an airtight housing, and gas is introduced into the unit to be compressed and discharged thereby. The scroll compressing unit comprises a pair of scroll members in lubricative contact with each other; a stationary scroll member fixed to the housing and a orbiting scroll member orbiting relatively to the stationary member. Each scroll member has a substantially circular end plate and a spiroidal wrap planted in one side of the end plate. Usually, the spiroidal configuration of the wrap is either an involute curve or a continuous curve determined by a plurality of circular arcs (that is series semi-circle curve), so that the respective wrap-side faces of the two scroll members are in contact with each other simultaneously at a plurality of contact areas when the scroll members are joined together. The respective wraps of the two scroll members have the same height so that the end faces of the wraps abut against the end plates opposite thereto. Thus, when the scroll members are combined together, a plurality of crescent-shaped compression chambers are defined by the wraps and the end plates.
When the scroll compressor is operated, the orbiting scroll member orbits relatively to the stationer scroll member, so that the sliding-contact portions between the side faces of the wraps move successively from the peripheral edge portion toward the central portion. As a result, the crescent-shaped compression chambers are reduced in capacity, so that the gas in the chambers is compressed. The compressed gas is discharged from the housing through a discharge port in the center of the end plate.
The end plate and the wrap are made from forgings or die castings, by machining the forgings or die castings by means of an NC cutting machine into products having predetermined sizes. This method of manufacturing the end plate and the wrap requires great cost, and cannot be employed to manufacture the end plate and the wrap in large quantities. This is why the use of this method has been limited to the manufacture of the end plates and the wraps for use in large-capacity compressors.
To manufacture the end plate of the scroll member at low cost, the end plate can be made of steel or aluminum, and a spiroidal groove can be cut in one surface of the end plate. In this case, the wrap is fitted in the spiroidal groove, thereby forming the scroll member. However, this method has a problem. The wraps of both scroll members must be in oil-sealed contact, and the end of either wrap must also be in oil-sealed contact with the end plate. For example, the gap between the contacting sides of the wraps must be reduced to 10 .mu.m or less, in order that the gas leakage from the compression chamber is minimized and, hence, the compressor has a sufficient compression efficiency. Therefore, the allowable tolerance of either scroll member must be 5 .mu.m or less, and the components of either scroll member must be scraped with an extremely high precision.
However, in above-mentioned method for manufacturing, it is hard to form the groove in the end plate with high accuracy, so that a desired working accuracy cannot be obtained. It is also difficult to fit the wrap accurately into the groove, so that the gap between the adjacent turns of the wrap is prone to errors. Accordingly, the mounting accuracy for the wrap becomes so low that the compressed gas is liable to leak from chambers, thus failing to enjoy high compressibility. In such scroll member of the wrap-fit type, moreover, the formation of the groove in the end plate requires much labor, thereby entailing increase in manufacturing cost.
In consideration of these circumstances, an improved forming technique has been developed. In this method, a wrap and an end plate are formed integrally, instead of being formed separately. Disclosed in Japanese Patent Disclosure No. 59-135309 is an integral forming method in which a scroll member is integrally formed by injection molding of synthetic resin using metallic mold. According to this method, a wrap and an end plate are formed integrally, so that the scroll member can be manufactured quickly. Since the scroll member has a complicated configuration, and is usually designed so that the wrap is thinner than the end plate, so the individual parts of the member are subject to differences in contraction rate during a process of solidification. Thus, the scroll member is warped, and the desired working accuracy cannot be obtained.
Disclosed in Japanese Utility Model Disclosure No. 60-8493, moreover, is a scroll member of a type such that a spiroidal core, which protrudes from one side of an end plate, is coated with resin by injection molding.
In the scroll member of this type, however, the resin is injected through a passage on the mold side, so that its finally solidifying portion is situated on the end-face (or top) side of a wrap. Due to differences in contraction rate, therefore, the core cannot be coated as required, so that the shape of the wrap, especially of its end face, is lowered in accuracy. Since an injection hole and a vent hole of a mold opens corresponding to the wrap end face burrs are formed on the end face, when these burrs are removed by scraping the precision of the wrap the end face, is impaired.
In general, the wrap end face is liable to wear when it slides on the other end plate. In order to prevent the gas from being by-passed, it is to be desired that no gap should exist between the contact portions of the wrap end face and the end plate. I the scroll member, furthermore, the working accuracy of wraps is so low that the wraps and the end plates wear much by unbalance sliding. Thus, the scroll member is lowered in life performance.